1. Field of the Invention
The present invention relates to a fabrication method of a solid electrolytic capacitor and more particularly, to a fabrication method of a solid electrolytic capacitor using an organic conducting polymer (for example, polypyrrole, polythiophene, polyaniline, or polyfuran) as a solid electrolyte of the capacitor, which decreases the leakage current.
2. Description of the Prior Art
In recent years, electronic components have been becoming miniaturized more and more and their operation speed and operation frequency have been becoming higher and higher. Also, digitalization has been progressing rapidly in signal processing. To cope with the tendency, there has been the strong need of decreasing the impedance of capacitors in high-frequency regions.
Typically, a solid electrolytic capacitor has a porous capacitor body or pellet serving as an anode, which is typically made by sintering a powder of a valve metal such as tantalum (Ta) and aluminum (Al). An oxide layer of the valve metal, which serves as a dielectric, is formed on the expanded surface of the porous capacitor body. A solid electrolyte layer serving as a cathode is formed on the oxide layer. An anode lead is fixed to the capacitor body to be electrically connected to the same through an anode wire. An electrically-conductive layer is formed on the solid electrolyte layer. A cathode lead is fixed to the solid electrolyte layer to be electrically connected to the same through the electrically-conductive layer.
The porous capacitor body, the oxide layer, the solid electrolyte layer, the electrically-conductive layer, the anode wire, the anode lead, and the cathode lead are encapsulated by a synthetic-resin package in such a way that an outer part of the anode lead and an outer part of the cathode lead protrude from the package.
The solid electrolyte layer has a function of electrically interconnecting the cathode lead with the entire surface of the dielectric (i.e., the oxide layer) formed on the capacitor body. Therefore, from this viewpoint, it is desirable that the solid electrolyte layer is a substance having a high electrical conductivity. On the other hand, the solid electrolyte layer needs to have a healing function of healing an electrical short-circuit due to defects in the dielectric.
Accordingly, a metal, which has a high electrical conductivity, but has no dielectric healing function, cannot be used as the solid electrolyte layer. As a result, conventionally, manganese dioxide (MnO.sub.2) or lead dioxide (PbO.sub.2) has been popularly used as the solid electrolyte layer, because MnO.sub.2 and PbO.sub.2 have a property that they are transformed from an electrical conductor into an electrical insulator due to the heat generated by a short-circuit current caused by the defects in the dielectric.
However, MnO.sub.2 and PbO.sub.2 have a problem of a comparatively low electrical conductivity of approximately 0.1 S/cm, resulting in a high Equivalent Series Resistance (ESR) in high-frequency regions.
To solve this problem of the high ESR, recently, various capacitors using one of conducting polymers such as polypyrrole as the solid electrolyte layer have been vigorously developed. This is because these conducting polymers have an electrical conductivity as high as 10 to 100 S/cm.
It has been known that these conducting polymer layers can be formed on the oxide layer with the use of an "electrolytic polymerization" or "chemically-oxidative polymerization" process.
On the other hand, however, the conventional capacitors using one of the conducting polymers have a problem of a high leakage current compared with the popular capacitors using MnO.sub.2 or PbO.sub.2 as the solid electrolyte layer. To solve this problem, an improved fabrication method was developed, in which the oxide layer serving as the dielectric is subjected to "anodic reoxidation" (which maybe termed "reformation") using an organic electrolyte solution prepared by adding a small amount of phosphoric acid to methanol after the solid electrolyte layer is formed by one of the conducting polymer layers. This method is disclosed in the Japanese Non-Examined patent publication No. 8-64476 published in March 1996.
With the improved fabrication method of a solid electrolyte capacitor disclosed in the Japanese Non-Examined patent publication No. 8-64476, however, there is a problem that a leakage current of the oxide layer serving as the dielectric is yet high and the oxide layer tends to be decomposed by the large leakage current. This degrades the ESR characteristic of the capacitor.